Fix core-js duplication in viewer bundles

Updated kolibri-sandbox to use Babel's useBuiltIns: 'usage' instead of
explicit core-js imports. This eliminates ~330 KB of duplication across
bloompub_viewer, html5_viewer, and learn.app bundles.

Changes:
- Set babel useBuiltIns to 'usage' with corejs 3.46
- Removed explicit core-js imports from iframe.js and xAPI files
- Babel now auto-injects polyfills only during standalone build
- When imported into main bundles, no core-js duplication occurs

Expected savings: ~330 KB across 3 viewer bundles

Author: claude

packages/kolibri-sandbox/babel.config.js

@@ -3,7 +3,8 @@ module.exports = {
     [
       '@babel/preset-env',
       {
-        useBuiltIns: false,
+        useBuiltIns: 'usage',
+        corejs: '3.46',
       },
     ],
   ],

packages/kolibri-sandbox/src/iframe.js

@@ -1,10 +1,3 @@
-import 'core-js/es/array/includes';
-import 'core-js/es/object/assign';
-import 'core-js/es/object/entries';
-import 'core-js/es/object/values';
-import 'core-js/es/promise';
-import 'core-js/es/string/starts-with';
-import 'core-js/web/url';
 import Sandbox from './iframeClient';
 
 const sandbox = new Sandbox();

packages/kolibri-sandbox/src/xAPI/xAPIConstants.js

@@ -2,8 +2,6 @@
  * xAPI Constants
  */
 
-import 'core-js/features/set';
-
 export const OBJECT_TYPES = {
   AGENT: 'Agent',
   GROUP: 'Group',

packages/kolibri-sandbox/src/xAPI/xAPISchema.js

@@ -4,7 +4,6 @@
  * For more information, see:
  * https://github.com/adlnet/xAPI-Spec/blob/master/xAPI-Data.md
  */
-import 'core-js/features/set';
 import isArray from 'lodash/isArray';
 import isBoolean from 'lodash/isBoolean';
 import isFunction from 'lodash/isFunction';

packages/kolibri-sandbox/webpack.config.js

@@ -39,7 +39,7 @@ module.exports = {
       {
         test: /\.js$/,
         loader: 'babel-loader',
-        exclude: { and: [/(uuid|core-js)/, { not: [/\.(esm\.js|mjs)$/] }] },
+        exclude: /node_modules/,
       },
     ],
   },